1. Field of the Invention
The present invention relates to a catalyst for decreasing emissions contained in exhaust gas, such as hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NOx).
2. Description of the Related Art
Generally, an exhaust system for an engine is provided with a purifying catalyst for decreasing harmful emissions in exhaust gas from the engine. This purifying catalyst typically comprises an oxide support, and a catalytic noble metal, such as Pt, Pd and/or Rh, supported on the oxide support. This catalytic noble metal involves a problem about sintering which is more likely to occur along with a rise in exhaust gas temperature. Thus, there is a strong need for retaining the catalytic noble metal in a highly dispersed state to suppress the sintering even under a high-temperature atmosphere.
With a view to meeting the above need, there has been proposed an exhaust gas purifying catalyst which comprises a honeycomb-like substrate, and a catalytic layer formed on a cell wall of the substrate, as disclosed, for example, in Japanese Patent Laid-Open Publication No. 2004-174490. The catalytic layer includes an activated alumina, and an oxygen-absorbing material consisting of a mixed oxide (composite oxide) which contains Ce, Zr and Nd, and retains Rh as a catalytic noble metal in such a manner that Rh atoms are located at crystal lattice points or between the lattice points of the mixed oxide. In this purifying catalyst, the catalytic noble metal is strongly bonded to the mixed oxide and restricted in its migration even under a high-temperature atmosphere because of retaining Rh atoms as the catalytic noble metal at crystal lattice points or between the lattice points of the mixed oxide, and thus, the occurrence of sintering is effectively suppress and thereby provide enhanced heat resistance. In addition, the catalytic noble metal can be retained in a dispersed manner so as to enhance conversion efficiency of exhaust emissions.
Japanese Patent Laid-Open Publication No. 2005-161143 also discloses the same type of exhaust gas purifying catalyst as that in the Japanese Patent Laid-Open Publication No. 2004-174490, which has a catalytic noble metal retained such that atoms of the noble metal are located at crystal lattice points or between the lattice points of a mixed oxide.
From the perspective of improving air environment, there has been a need for further enhanced conversion efficiency of exhaust emissions in recent years. In thus situation, while the purifying catalyst having one kind of mixed oxide retaining a catalytic noble metal, as disclosed in the above publications, can enhance conversion efficiency in some extent by suppress the occurrence of sintering and enhance heat resistance, it has a certain limit of improvement in its conversion efficiency.
In view of the above problem, it is an object of the present invention to provide a purifying catalyst capable of achieving further enhanced conversion efficiency of exhaust emissions.